1. Field of the Invention
The present invention pertains to an inexpensive method and apparatus for recording a scrambled video signal and playing back the video signal in descrambled format within a secure environment.
2. Description of the Prior Art
This invention relates to an inexpensive method and apparatus for scrambling and descrambling a video signal. More particularly, this invention relates to an improved arrangement for scrambling the video and audio signals, adding an authorization code to the audio and playing back descrambled video and audio signals when authorized within a secure environment.
Scrambling of television signals has been done in many ways. Some of these techniques are recordable and some are not. Each of these techniques generally suffer from the problem of access to the descrambled image for subsequent recording. Typical scrambling techniques capable of being recorded are described below. In each of these techniques significant video processing is required.
A very simple technique of video scrambling of a recorded signal is taught by Perrett in U.S. Pat. No. 4,030,128 ('128) issued Jun. 14, 1977, and incorporated by reference herein. The '128 patent discloses the concept of modifying the normal recording format such that the "control track" and one "audio track" are reversed. See FIG. 2. When such a recording is played back on a normal machine the audio signal would be connected to the control track circuitry in the playback machine. As is well known in the art of video recording, the control track of a helical scan recording provides the control of the tape and rotating head drum speeds to provide a synchronized playback of the recording. Without such a proper control signal the playback signal from the playback recording is generally unviewable. However, the development of the '128 patent was prior to extensive use of pause and still playback in helical scan recorders. Even with the control track reversal a viewer can observe certain graphic images such as charts and fixed material while in the still frame or pause mode. The '128 patent's usefulness generally is limited to the loss of entertainment value perceived in a moving picture and does not prevent a viewer from discerning information on a frame by frame basis. Such a system would have limited usefulness in training or corporate information tapes where information security is important.
Another technique commonly used to obscure the visibility of a video signal is to invert the polarity of the video signal. As is well known in the art of video engineering most video signals use the sync negative concept. As shown in FIG. 1, the synchronizing portion is contained from the blanking level downward. The video level is in the upper portion and an increase in the video signal represents an increase in brightness of the image.
More secure recordable scrambling and descrambling systems are known in the art, but are much more expensive to implement.
One recordable scrambling and descrambling method is described in U.S. Pat. No. 4,916,736 ('736) by John O. Ryan issued on Apr. 10, 1990, incorporated by reference. The '736 patent describes a method and apparatus for time shifting the active portion of video signals towards and away from the line timing reference portion in a pseudo-random fashion prior to recording the signal onto a tape and transmitting it to the user. The signals are descrambled using a reverse time shifting technique. Since the video signal has to be advanced or delayed precisely while maintaining full video quality, the '736 system requires an expensive descrambling process.
Another recordable scrambling and descrambling method is described in a series of patents. These include U.S. Pat. Nos. 5,058,157 issued Oct. 15, 1991, 5,438,620 issued Aug. 1, 1995, 5,579,390 issued Nov. 26, 1996 and 5,581,307 issued Dec. 3, 1996, all incorporated by reference. These patents describe a method and apparatus for time shifting the active portion of the signal relative to the line timing reference similar to the '736 patent and are colloquially known as PhaseKrypt.RTM. patents. However, these patents describe several improvements over the '736 patent including improved encoding, less expensive decoding, overlay signals and edge fill techniques. With these improvements, PhaseKrypt encoded video offers dramatic loss of entertainment value but does not produce sufficient concealment to prevent charts and graphs from being read. Further more, when playback is paused there is still less concealment.
It is known in the art to combine a television receiver/monitor and a video cassette recorder to provide a convenient one piece package. Most of these combination packages have a single tuner and no video outputs.